1. Technical Field
The invention relates to transparent thin films that are electrical conductors. More particularly, the invention relates to conductive films that are transparent in the visible and/or infrared regions of the electromagnetic spectrum.
2. Description of the Prior Art
Transparent electrical conductors, generally configured as thin films, are widely used for purposes such as providing electrodes for electro-optical displays (e.g., LCD and electro-chromic panels) and in architectural glazing that may be heat wave rejecting or that may be heated by passing electrical current through the coating to heat window glass. The art recognizes heat wave reflective and electrically conductive, substantially transparent sheets, such as glazing sheets, which include a transparent substrate and one or more sequential transparent silver layers supported by a substrate. See, for example, U.S. Pat. No. 6,040,939. The art also recognizes visibly transparent, infrared reflective sheets for use in heat reflective architectural glazing. These may comprise a transparent solid substrate and one or more sequential transparent gold-clad silver layers supported by a substrate. An example of such a film sandwich has been described by the inventor in U.S. Pat. No. 6,255,003.
It is also known to make coated articles of manufacture that comprise a substantially transparent substrate with a silver-based substantially transparent dual-function coating on a surface of the substrate. See, for example, U.S. Pat. No. 6,040,939. Other types of heat wave reflective and electrically conductive comprise transparent conductive oxides (hereinafter TCO). TCO coatings are wide band gap oxides that do not include silver or other metal layers. The art further recognizes that both silver-based and TCO type transparent electrically conductive coatings are only transparent in the visible spectrum and are highly reflective in the infrared (IR) spectral region. TCO substances include, but are not limited to mixtures of indium oxide doped with tin [In2O3:Sn] which is commonly referred to as ITO and which may comprise less oxygen (generally referred to as ITOx); and zinc oxide doped with aluminum [ZnO:Al], which is commonly referred to as ZAO.
The prior art provides layers evidencing two patterns, the single system layered: oxide-silver-metal buffer-oxide; and the split system layered: oxide-silver-metal buffer-oxide-silver-metal buffer-oxide. Specifically, U.S. Pat. No. 6,040,939 follows this pattern, using silicon as the semiconductor buffer layer. Within these layered structures, the oxide layers provide important anti-reflective (AR) functions. The important properties of AR coatings include transmittance enhancement and protection of the silver layer. High electrical conductance requires minimal electron scattering loss at the interfaces of conductive film. Therefore, electron scattering must be minimized through implementation of non-scattering interfaces at both sides of the silver film. Moreover, what is not known in the art is the provision of a visual and/or infrared transparent electrical conductor. It would therefore be advantageous to provide a visual and/or infrared transparent, electrically conductive thin film system.
Optical systems are sometimes provided with an electrically heated window in order to dispel condensed moisture—i.e., as a defroster. For visible light systems this can be provided by means of a transparent conductive film. Infrared optical systems would be well served by the availability of an infra-red transparent, electrically conductive film.